Block vector prediction in intra block copy mode

ABSTRACT

Aspects of the disclosure provide methods and apparatuses for video encoding/decoding. An apparatus for video decoding includes processing circuitry that decodes prediction information for a current block in a current coded picture. The prediction information indicates an intra block copy mode and a first block vector predictor (BVP) used for the current block. The first BVP points to a first reference block (RB). The processing circuitry determines whether the first RB is accessible from a memory. When the first RB is determined to be inaccessible from the memory, the processing circuitry determines a second RB that is accessible from the memory according to at least one of a default RB, the first RB, and the current block. The processing circuitry includes a second BVP in a BVP candidate list. The second BVP points to the second RB. The processing circuitry reconstructs the current block based on the BVP candidate list.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/698,010, “TECHNIQUES FOR BLOCK VECTORPREDICTION IN INTRA PICTURE BLOCK COMPENSATION” filed on Jul. 13, 2018,and U.S. Provisional Application No. 62/791,866, “BLOCK VECTORPREDICTION IN INTRA PICTURE BLOCK COMPENSATION” filed on Jan. 13, 2019,which are incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate) of, for example, 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or maybe predicted itself.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEMNVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (105) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1C, a current block (111) can include samples thathave been found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (112 through 116, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes receiving circuitry and processing circuitry.

The processing circuitry decodes prediction information for a currentblock in a current coded picture that is a part of a coded videosequence. The prediction information indicates an intra block copy (IBC)mode and a first block vector predictor (BVP) used for the currentblock. The first BVP points to a first reference block (RB). The processcircuitry determines whether the first RB is accessible from a memory.When the first RB is determined to be inaccessible from the memory, theprocessing circuitry determines a second RB that is accessible from thememory according to at least one of a default RB, a predefined pixelvalue, the first RB, and the current block. The processing circuitryincludes a second BVP in a BVP candidate list, where the second BVPpoints to the second RB. The processing circuitry reconstructs thecurrent block based on the BVP candidate list.

In an embodiment, when an allowed reference area of the current block isaccessible from the memory, the processing circuitry determines whetherthe first RB is outside the allowed reference area of the current block.When the first RB is determined to be outside the allowed reference areaof the current block, the processing circuitry indicates that the firstRB is inaccessible from the memory. In an example, the allowed referencearea of the current block includes a reconstructed area in a currentcoding tree unit (CTU). In another example, the allowed reference areaof the current block includes the reconstructed area in the current CTUand a part of an adjacent CTU that is on a left side of the current CTU.

The processing circuitry can determine whether at least one available RBis found in the allowed reference area of the current block. When atleast one available RB is found in the allowed reference area of thecurrent block, the processing circuitry determines the second RBaccording to one of the at least one available RB that is nearest to thefirst RB. When at least one available RB is not found in the allowedreference area of the current block, the processing circuitry determinesthe second RB according to the default RB or the predefined pixel value.When a coordinate of the first BVP is (x, y), a top-left coordinate ofthe default RB is one of an origin point (0, 0) of the current picture,a boundary point (x, 0) of the current picture that has the samehorizontal coordinate as the first BVP, and a boundary point (0, y) ofthe current picture that has the same vertical coordinate as the firstBVP.

In an embodiment, when a constrained reference area of the current blockis inaccessible from the memory, the processing circuitry determineswhether the first RB is inside the constrained reference area of thecurrent block. When the first RB is determined to be inside theconstrained reference area of the current block, the processingcircuitry indicates that the first RB is inaccessible from the memory.

When the first RB is determined to be inside the constrained referencearea of the current block, the processing circuitry determines thesecond RB to be outside and close to the constrained reference areaaccording to a size of the constrained reference area and a size of thecurrent block. The size of the constrained reference area is a multipleof a size of CTU.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes.

FIG. 1B is an illustration of exemplary intra prediction directions.

FIG. 1C is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 shows an example of the intra block copy (IBC) prediction mode,according to an embodiment of the disclosure.

FIGS. 9A-9D show an example of an updating process for an allowedreference area of the IBC prediction mode, according to an embodiment ofthe disclosure.

FIG. 10 shows an example of modifying a block vector according to anembodiment of the disclosure.

FIG. 11A shows an example of selecting a second reference block when afirst reference block is inside a constrained reference area accordingto an embodiment of the disclosure.

FIG. 11B shows another example of selecting a second reference blockwhen a first reference block is inside a constrained reference areaaccording to an embodiment of the disclosure.

FIG. 12 shows a flow chart outlining an exemplary process according toan embodiment of the disclosure.

FIG. 13 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick, and the like.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values that canbe input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation that the intra prediction unit (452) has generated to theoutput sample information as provided by the scaler/inverse transformunit (451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector allowed reference area,and so forth. The controller (550) can be configured to have othersuitable functions that pertain to the video encoder (503) optimized fora certain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415) andthe parser (420) may not be fully implemented in the local decoder(533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quad-tree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

In general, block based compensation based on a different picture may bereferred to as motion compensation or inter prediction blockcompensation. However, block compensation may be done from a previouslyreconstructed area within a same picture. Such block compensation may bereferred to as intra picture block compensation, current picturereferencing (CPR), or intra block copy (IBC).

Aspects of the disclosure provide techniques for block basedcompensation within a same picture (e.g., IBC prediction mode).

According to aspects of the disclosure, in the IBC prediction mode, adisplacement vector that indicates an offset between a current block anda reference block within the same picture is referred to as a blockvector (BV). It is noted that the reference block is alreadyreconstructed prior to the current block. In addition, for parallelprocessing, a reference area that is at a tile/slice boundary orwave-front ladder shape boundary may be excluded from being used as anavailable reference block. Due to these constraints, a block vector maybe different from a motion vector that can be at any value (positive ornegative, at either x or y direction) in motion compensation of theinter prediction mode.

The coding of a block vector in the IBC prediction mode can be eitherexplicit or implicit. In the explicit mode, a block vector differencebetween a block vector and a predictor of the block vector is signaled.A way of coding a block vector in the explicit mode of the IBCprediction mode may be similar to a way of coding a motion vector inadvanced motion vector prediction (AMVP) mode of the inter predictionmode. In the implicit mode, a block vector is recovered from a predictorof the block vector without using a block vector difference, in asimilar way as a motion vector prediction in merge mode of the interprediction mode. In addition, the resolution of a block vector may berestricted to integer positions in one embodiment but may be allowed topoint to fractional positions in another embodiment.

The use of the IBC prediction mode at the block level can be signaledusing, for example, a block level flag (referred to as an IBC flag) or areference index. When using the IBC flag, the current block may not becoded in implicit mode. When using the reference index, the currentdecoded picture can be treated as a reference picture that is put in alast position of a reference picture list. This reference picture mayalso be managed together with other temporal reference pictures in adecoded picture buffer (DPB).

FIG. 8 shows an example of the intra block copy (IBC) prediction mode,according to an embodiment of the disclosure. In the example of FIG. 8,a current picture (800) is being reconstructed and includes areconstructed area (801) (gray area) and a to-be-decoded area (802)(white area). The blocks in the reconstructed area (801) are alreadydecoded and the blocks in the to-be-reconstructed area (802) are eitherbeing decoded or to-be-decoded. A current block (804) is in theto-be-reconstructed area (802) and being decoded. The current block(804) can be decoded from a reference block (805) that is in thereconstructed area (801). The decoding of the current block (804) isbased on a block vector (803) that is an offset between the currentblock (804) and the reference block (805).

According to aspects of the disclosure, a reference block (e.g.,reference block (805)) used to derive a block vector (e.g., block vector(803)) for a current block (e.g., current block (804)) is within anallowed reference area of the IBC prediction mode. When the allowedreference area is accessible from a memory with a limited memory space,such as an on-chip memory, the allowed reference area may be constrainedwithin a certain area.

According to some embodiments, an allowed reference area of the IBCprediction mode is constrained to be within a current CTU where acurrent block resides. In an example, a memory to store referencesamples for the allowed reference area in the IBC prediction mode is 1CTU size (e.g., 128×128 samples). If 1 CTU size (128×128 samples)includes four regions with each region having 64×64 samples, the memorymay store such four regions, in which one region may be currentlyreconstructed 64×64 samples and the other three regions with each regionhaving 64×64 samples may be reference samples.

According to some embodiments, an allowed reference area of the IBCprediction mode can be extended to some parts of a left CTU of thecurrent CTU while keeping the memory to store the allowed reference areaunchanged (e.g., 1 CTU size), so that the allowed reference area may notbe constrained to be within the current CTU. It is noted that theallowed reference area may depend on a position of a current block inthe current CTU. That is, the allowed reference area may be updatedaccording to the position of the current block in the current CTU.

FIGS. 9A-9D show an example of an updating process for an allowedreference area of the IBC prediction mode, according to an embodiment ofthe disclosure. As described above, the effective allowed reference areais extended to some parts of a left CTU (910) of a current CTU (900).

During this updating process, the stored reference samples from the leftCTU are updated with the reconstructed samples from the current CTU. InFIGS. 9A-9D, gray color regions indicate already reconstructed regions,white color regions indicate to-be-reconstructed regions, and regionswith vertical stripes and text “Curr” indicate current coding/decodingregions where current blocks reside. In addition, in each figure, theleft four regions (911)-(914) belong to the left CTU (910) and the rightfour regions (901)-(904) belong to the current CTU (900).

It is noted that all of four regions (911)-(914) of the left CTU (910)are already reconstructed. Thus, the memory initially stores all ofthese four regions of reference samples from the left CTU (910), andthen updates a region of reference samples from the left CTU (910) witha same relative region of currently reconstructed samples from thecurrent CTU (900).

For example, in FIG. 9A, a current region (901) in the current CTU (900)is under reconstruction, and a co-located region in the left CTU (910)of the current region (901) is an already reconstructed region (911).The co-located region (911) is in a region of the left CTU (910) withthe same relative region as the current block (901) in the current CTU(900). Thus, the memory region that stores reference samples of theco-located region (911) is updated to store the reconstructed samples ofthe current region (901), and an “X” is marked in the co-located region(911) in FIG. 9A to indicate that the reference samples of theco-located region (911) are no longer stored in the memory.

Similarly, in FIG. 9B, a current region (902) in the current CTU (900)is under reconstruction, and a co-located region in the left CTU (910)of the current region (902) is a region (912). The co-located region(912) is in a region of the left CTU (910) with the same relative regionas the current region (902) in the current CTU (900). Thus, the memoryregion that stores reference samples of the co-located region (912) isupdated to store the reconstructed samples of the current region (902),and an “X” is marked in the co-located region (912) in FIG. 9B toindicate that the reference samples of the co-located region (912) areno longer stored in the memory.

In FIG. 9C, a current region (903) in the current CTU (900) is underreconstruction, and a co-located region in the left CTU (910) of thecurrent region (903) is a region (913). The co-located region (913) isin a region of the left CTU (910) with the same relative region as thecurrent region (903) in the current CTU (900). Thus, the memory regionthat stores reference samples of the co-located region (913) is updatedto store the reconstructed samples of the current region (903), and an“X” is marked in the co-located region (913) in FIG. 9C to indicate thatthe reference samples of the co-located region (913) are no longerstored in the memory.

In FIG. 9D, a current region (904) in the current CTU (900) is underreconstruction, and a co-located region in the left CTU (910) of thecurrent region (904) is a region (914). The co-located region (914) isin a region of the left CTU (910) with the same relative region as thecurrent region (904) in the current CTU (900). Thus, the memory regionthat stores reference samples of the co-located region (914) is updatedto store the reconstructed samples of the current region (904), and an“X” is marked in the co-located region (914) in FIG. 9D to indicate thatthe reference samples of the co-located region (914) are no longerstored in the memory.

According to aspects of the disclosure, when a first reference block isoutside an allowed reference area, a block vector pointing to the firstreference block is modified such that the modified block vector pointsto a second reference block that is inside the allowed reference areaand nearest to the first reference block.

FIG. 10 shows an example of modifying a block vector according to anembodiment of the disclosure. In FIG. 10, a current CTU (1000) is anallowed reference area of a current block (1001). Therefore, a left CTU(1010), an above CTU (1020), an above-left CTU (1030), and anabove-right CTU (1040) are considered to be outside the allowedreference area in FIG. 10.

In an embodiment, when a block vector (1012) points to a reference block(1011) in the left CTU (1010), the block vector (1012) is considered tobe not valid. Therefore, the block vector (1012) is clipped by reducingits horizontal dimension to a vertical boundary of the current CTU(1000). The clipped block vector (1013) points to a reference block(1002) that is inside the current CTU (1000) and nearest to thereference block (1011).

In an embodiment, when a block vector (1022) points to a reference block(1021) in the above CTU (1020), the block vector (1022) is considered tobe not valid. Therefore, the block vector (1022) is clipped by reducingits vertical dimension to a horizontal boundary of the current CTU(1000). The clipped block vector (1023) points to a reference block(1003) that is inside the current CTU (1000) and nearest to thereference block (1021).

In an embodiment, when a block vector (not shown) points to a referenceblock (1031) in the above-left CTU (1030), the block vector isconsidered to be not valid. Therefore, the block vector is clipped byreducing its both horizontal dimension and vertical dimension to anupper-left corner of the current CTU (1000). The clipped block vector(not shown) points to a reference block (1004) that is inside thecurrent CTU (1000) and nearest to the reference block (1031).

In an embodiment, when a block vector (not shown) points to a referenceblock (1041) in the above-right CTU (1040), the block vector isconsidered to be not valid. Therefore, the block vector is clipped byreducing its both horizontal dimension and vertical dimension to anupper-right corner of the current CTU (1000). The clipped block vector(not shown) points to a reference block (1005) that is inside thecurrent CTU (1000) and nearest to the reference block (1041).

According to aspects of the disclosure, when a reference block isoutside an allowed reference area, a block vector pointing to thereference block is modified such that a horizontal dimension or avertical dimension of the modified block vector is within a valid searchrange.

When a horizontal clipping is used, the horizontal dimension of theblock vector can be clipped such that the horizontal dimension of themodified block vector is bounded between a left boundary of the validsearch range and a right boundary of the valid search range.

When a vertical clipping is used, the vertical dimension of the blockvector can be clipped such that the vertical dimension of the modifiedblock vector is bounded between a top boundary of the valid search rangeand a bottom boundary of the valid search range.

According to aspects of the disclosure, when a reference block isoutside an allowed reference area and there is no available referenceblock inside the allowed reference area, a default reference block isused for predicting a current block. In an embodiment, the defaultreference block can be located at (0, 0), i.e., an origin point of acurrent picture.

In an embodiment, a location of the default reference block depends on ablock vector pointing to the invalid reference block. If a coordinate ofthe block vector is (x, y), the default reference block locates at aboundary point (x, 0) or (0, y) of the current picture. The boundarypoint (x, 0) has the same horizontal coordinate as the block vector, andthe boundary point (0, y) has the same vertical coordinate as the blockvector. When a reference block is outside an allowed reference area, ablock vector pointing to the reference block is not put into a blockvector predictor candidate list for predicting a current block.

According to aspects of the disclosure, when a reference block isoutside an allowed reference area and there is no available referenceblock inside the allowed reference area, a predefined pixel value isused for predicting a current block. In an embodiment, the predefinedpixel value can be a median value of a bit depth. For example, when thebit depth is 10 bit, the median value and the predefined pixel value canbe 512.

According to aspects of the disclosure, a reference block (e.g.,reference block (805)) used to derive a block vector (e.g., block vector(803)) for a current block (e.g., current block (804)) is outside aconstrained reference area of the IBC prediction mode. For example, whenan immediate neighborhood of a current block is used as a reference areaof the current block and stored in a memory with a high latency, such asan off-chip memory, a cost of accessing the immediate neighborhood asthe reference area to predict the current block may be too expensive interms of hardware design. In such an example, the immediate neighborhoodmay be considered as the constrained reference area of the IBCprediction mode. Therefore, when a constrained reference area isspecified in an IBC prediction mode, the current block may refer to areference block that is outside the constrained reference area.

According to aspects of the disclosure, when a current block refers to afirst reference block that is inside a constrained reference area in anIBC prediction mode, a second reference block that is outside and closeto the constrained reference area is used for prediction the currentblock. A location of the second reference block depends on a size of theconstrained reference area and a size of the current block. It is notedthe size of the constrained reference area is a multiple of a size ofCTU.

FIG. 11A shows an example of selecting a second reference block when afirst reference block is inside a constrained reference area accordingto an embodiment of the disclosure. In the FIG. 11A example, a currentCTU (1100) is the constrained reference area of a current block (1101).Therefore, when the first reference block is inside the current CTU(1100), the second reference block may locate outside a verticalboundary or a horizontal boundary of the current CTU (1100).

In the FIG. 11A example, a top-left coordinate of the current CTU (1100)is (CTUx, CTUy) and a top-left coordinate of the current block (1101) is(CUx, CUy). A width and a height of the current block (1101) are CUwidthand CUheight, respectively.

In an embodiment, the second reference block locates left to thevertical boundary of the current CTU (1100) and has the same verticalcoordinate with the current block (1101). In an example, the secondreference block is (1102) and a corresponding block vector is (CTUx−CUx−CUwidth, 0). In another example, the reference block is (1103) anda corresponding block vector is (CTUx−CUx−2*CUwidth, 0).

The second reference block can be located above the horizontal boundaryof the current CTU (1100) and has the same horizontal coordinate withthe current block (1101). In an example, the second reference block is(1104) and a corresponding block vector is (0, CTUy −CUy−CUheight). Inanother example, the second reference block is (1105) and acorresponding block vector is (0, CTUy−CUy−2*CUheight).

The second reference block can be located outside an upper-left cornerof the current CTU (1100). In an example, the second reference block is(1106) and a corresponding block vector is (CTUx−CUx−CUwidth,CTUy−CUy−CUheight). In another example, the second reference block is(1107) and a corresponding block vector is (CTUx−CUx−CUwidth, CTUy−CUy).In another example, the second reference block is (1108) and acorresponding block vector is (CTUx−CUx, CTUy−CUy−CUheight).

FIG. 11B shows another example of selecting a second reference blockwhen a first reference block is inside a constrained reference areaaccording to an embodiment of the disclosure. In the FIG. 11B example,both a current CTU (1110) and a left CTU (1130) are the constrainedreference area of a current block (1111). Therefore, when the firstreference block is inside the current CTU (1110) or the left CTU (1130),the second reference block may locate outside a horizontal boundary ofthe current CTU (1110) or a vertical boundary of the left CTU (1130).

In the FIG. 11B example, a top-left coordinate of the current CTU (1110)is (CTUx, CTUy) and a top-left coordinate of the current block (1111) is(CUx, CUy). A width and a height of the left CTU (1130) are CTUwidth andCTUheight, respectively. A width and a height of the current block(1111) are CUwidth and CUheight, respectively. It is noted thatCTUx<=CUx<CTUx+CTUwidth and CTUy<=CUy<CTUy+CTUheight.

In an embodiment, the second reference block can be located left to thevertical boundary of the left CTU (1130) and has the same verticalcoordinate with the current block (1111). In an example, the secondreference block is (1112) and a corresponding block vector is(CTUx−CTUwidth−CUx−CUwidth, 0). In another example, the reference blockis (1113) and a corresponding block vector is(CTUx−CTUwidth−CUx−2*CUwidth, 0).

In an embodiment, the second reference block can be located above thehorizontal boundary of the current CTU (1110) and has the samehorizontal coordinate with the current block (1111). In an example, thesecond reference block is (1114) and a corresponding block vector is (0,CTUy−CUy−CUheight). In another example, the second reference block is(1115) and a corresponding block vector is (0, CTUy−CUy−2*CUheight).

The second reference block can be located outside an upper-left cornerof the left CTU (1130). In an example, the second reference block is(1116) and a corresponding block vector is (CTUx−CTUwidth−CUx−CUwidth,CTUy−CUy−CUheight). In another example, the second reference block is(1117) and a corresponding block vector is (CTUx −CTUwidth−CUx−CUwidth,CTUy−CUy). In another example, the second reference block is (1118) anda corresponding block vector is (CTUx−CTUwidth−CUx, CTUy−CUy −CUheight).

The second reference block can be located outside an upper-left cornerof the current CTU (1110). In an example, the second reference block is(1119) and a corresponding block vector is (CTUx−CUx−CUwidth,CTUy−CUy−CUheight). In another example, the second reference block is(1120) and a corresponding block vector is (CTUx−CUx, CTUy−CUy−CUheight).

It is noted that a zero block vector may not be a valid block vector inthe IBC prediction mode since a block vector difference generated fromthe zero block vector may be quite large.

According to aspects of the disclosure, a valid block vector pointing toa valid reference block can be put into a merge candidate list as amerge candidate. When a merge candidate index indicates that the validblock vector is selected, the current block is implicitly indicated tobe coded in the IBC prediction mode. In one embodiment, the valid blockvector can be put in a last position of the merge candidate list.

Alternatively, the valid block vector can be put before or after acertain merge candidate in the merge candidate list. In one example, thevalid block vector can be put after the spatial merge candidates. Inanother example, the valid block vector can be put before zero mergecandidates. When more than one block vector is put into the mergecandidate list, an order of these block vector is not constrained.

It is noted that a block vector may not be used as a merge candidatewhen the block vector points to a reference block outside a currentpicture, a current tile, a current slice, or an allowed reference areafor the IBC prediction mode.

FIG. 12 shows a flow chart outlining an exemplary process (1200)according to an embodiment of the disclosure. The process (1200) can beused in the reconstruction of a block coded in an IBC prediction mode,so as to generate a prediction block for a block under reconstruction.In various embodiments, the process (1200) are executed by processingcircuitry, such as the processing circuitry in the terminal devices(210), (220), (230) and (240), the processing circuitry that performsfunctions of the video encoder (303), the processing circuitry thatperforms functions of the video decoder (310), the processing circuitrythat performs functions of the video decoder (410), the processingcircuitry that performs functions of the intra prediction module (452),the processing circuitry that performs functions of the video encoder(503), the processing circuitry that performs functions of the predictor(535), the processing circuitry that performs functions of the intraencoder (622), the processing circuitry that performs functions of theintra decoder (772), and the like. In some embodiments, the process(1200) is implemented in software instructions, thus when the processingcircuitry executes the software instructions, the processing circuitryperforms the process (1200).

The process (1200) may generally start at step (S1210), where theprocess (1200) decodes prediction information for a current block in acurrent coded picture that is a part of a coded video sequence. Theprediction information indicates an intra block copy (IBC) mode and afirst block vector predictor (BVP) used for the current block. The firstBVP points to a first reference block (RB). After decoding theprediction information, the process (1200) proceeds to step (S1220).

At step (S1220), the process (1200) determines whether the first RB isaccessible from a memory. In an embodiment, when an allowed referencearea of the current block is accessible from the memory, the process(1200) determines whether the first RB is outside the allowed referencearea of the current block. When the first RB is determined to be outsidethe allowed reference area of the current block, the process (1200)indicates that the first RB is inaccessible from the memory. In anexample, the allowed reference area of the current block includes areconstructed area in a CTU. In another example, the allowed referencearea of the current block includes the reconstructed area in the currentCTU and a part of an adjacent CTU that is on a left side of the currentCTU.

In an embodiment, when a constrained reference area of the current blockis inaccessible from the memory, the process (1200) determines whetherthe first RB is inside the constrained reference area of the currentblock. When the first RB is determined to be inside the constrainedreference area of the current block, the process (1200) indicates thatthe first RB is inaccessible from the memory. When the first RB isdetermined to be inaccessible from the memory, the process proceeds tostep (S1230).

At step (S1230), the process (1200) determines a second RB that isaccessible from the memory according to at least one of a default RB,the first RB, and the current block. In an embodiment, when the first RBis determined to be outside an allowed reference area of a currentblock, the process (1200) determines whether at least one available RBis found in the allowed reference area of the current block. When atleast one available RB is found in the allowed reference area of thecurrent block, the process (1200) determines the second RB according toone of the at least one available RB that is nearest to the first RB.Otherwise, when at least one available RB is not found in the allowedreference area of the current block, the process (1200) determines thesecond RB according to a default RB. When a coordinate of the first BVPis (x, y), a top-left coordinate of the default RB is one of an originpoint (0, 0) of the current picture, a boundary point (x, 0) of thecurrent picture that has the same horizontal coordinate as the firstBVP, and a boundary point (0, y) of the current picture that has thesame vertical coordinate as the first BVP.

In an embodiment, when the first RB is determined to be inside theconstrained reference area of the current block, the process (1200)determines the second RB to be outside and close to the constrainedreference area according to a size of the constrained reference area anda size of the current block. The size of the constrained reference areais a multiple of a size of CTU.

After determining the second RB, the process (1200) proceeds to step(1240). At step (S1240), the process (1200) includes a second BVP in aBVP candidate list. The second BVP points to the second RB. Then theprocess (1200) proceeds to step (1250).

At step (S1250), the process (1200) reconstructs the current block basedon the BVP candidate list. After reconstructing the current block, theprocess (1200) terminates.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 13 shows a computersystem (1300) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 13 for computer system (1300) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1300).

Computer system (1300) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1301), mouse (1302), trackpad (1303), touchscreen (1310), data-glove (not shown), joystick (1305), microphone(1306), scanner (1307), camera (1308).

Computer system (1300) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1310), data-glove (not shown), or joystick (1305), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1309), headphones(not depicted)), visual output devices (such as screens (1310) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1300) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1320) with CD/DVD or the like media (1321), thumb-drive (1322),removable hard drive or solid state drive (1323), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1300) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1349) (such as, for example USB ports of thecomputer system (1300)); others are commonly integrated into the core ofthe computer system (1300) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1300) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1340) of thecomputer system (1300).

The core (1340) can include one or more Central Processing Units (CPU)(1341), Graphics Processing Units (GPU) (1342), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1343), hardware accelerators for certain tasks (1344), and so forth.These devices, along with Read-only memory (ROM) (1345), Random-accessmemory (1346), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1347), may be connectedthrough a system bus (1348). In some computer systems, the system bus(1348) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1348),or through a peripheral bus (1349). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1341), GPUs (1342), FPGAs (1343), and accelerators (1344) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1345) or RAM (1346). Transitional data can be also be stored in RAM(1346), whereas permanent data can be stored for example, in theinternal mass storage (1347). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1341), GPU (1342), massstorage (1347), ROM (1345), RAM (1346), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1300), and specifically the core (1340) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1340) that are of non-transitorynature, such as core-internal mass storage (1347) or ROM (1345). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1340). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1340) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1346) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1344)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

APPENDIX A: ACRONYMS

-   AMVP: Advanced Motion Vector Prediction-   ASIC: Application-Specific Integrated Circuit-   BMS: Benchmark Set-   BV: Block Vector-   CANBus: Controller Area Network Bus-   CD: Compact Disc-   CPR: Current Picture Referencing-   CPUs: Central Processing Units-   CRT: Cathode Ray Tube-   CTBs: Coding Tree Blocks-   CTUs: Coding Tree Units-   CU: Coding Unit-   DPB: Decoder Picture Buffer-   DVD: Digital Video Disc-   FPGA: Field Programmable Gate Areas-   GOPs: Groups of Pictures-   GPUs: Graphics Processing Units-   GSM: Global System for Mobile communications-   HEVC: High Efficiency Video Coding-   HRD: Hypothetical Reference Decoder-   IBC: Intra Block Copy-   IC: Integrated Circuit-   JEM: Joint Exploration Model-   LAN: Local Area Network-   LCD: Liquid-Crystal Display-   LTE: Long-Term Evolution-   MV: Motion Vector-   OLED: Organic Light-Emitting Diode-   PBs: Prediction Blocks-   PCI: Peripheral Component Interconnect-   PLD: Programmable Logic Device-   PUs: Prediction Units-   RAM: Random Access Memory-   ROM: Read-Only Memory-   SCC: Screen Content Coding-   SEI: Supplementary Enhancement Information-   SNR: Signal Noise Ratio-   SSD: Solid-state Drive-   TUs: Transform Units-   USB: Universal Serial Bus-   VUI: Video Usability Information-   VVC: Versatile Video Coding

What is claimed is:
 1. A method for video decoding in a decoder, themethod comprising: decoding prediction information for a current blockin a current coded picture that is a part of a coded video sequence, theprediction information indicating an intra block copy (IBC) mode and afirst block vector predictor (BVP) used for the current block, the firstBVP pointing to a first reference block (RB); determining whether thefirst RB is accessible from a memory; when the first RB is determined tobe inaccessible from the memory, determining a second RB that isaccessible from the memory according to at least a median pixel value ofa bit depth of the coded video sequence; including a second BVP in a BVPcandidate list, the second BVP pointing to the second RB; andreconstructing the current block based on the BVP candidate list.
 2. Themethod of claim 1, wherein, when an allowed reference area of thecurrent block is accessible from the memory, the determining whether thefirst RB is accessible from the memory further comprises: determiningwhether the first RB is outside the allowed reference area of thecurrent block; and when the first RB is determined to be outside theallowed reference area of the current block, indicating that the firstRB is inaccessible from the memory.
 3. The method of claim 2, whereinthe allowed reference area of the current block includes one of: areconstructed area in a current coding tree unit (CTU); and thereconstructed area in the current CTU and a part of an adjacent CTU thatis on a left side of the current CTU.
 4. The method of claim 2, whereinthe determining the second RB further comprises: determining whether atleast one available RB is found in the allowed reference area of thecurrent block; when the at least one available RB is found in theallowed reference area of the current block, determining the second RBaccording to one of the at least one available RB that is nearest to thefirst RB; and when the at least one available RB is not found in theallowed reference area of the current block, determining the second RBaccording to a default RB or the predefined pixel value.
 5. The methodof claim 4, wherein, when a coordinate of the first BVP is (x, y), atop-left coordinate of the default RB is one of an origin point (0, 0)of the current coded picture, a boundary point (x, 0) of the currentcoded picture that has the same horizontal coordinate as the first BVP,and a boundary point (0, y) of the current coded picture that has thesame vertical coordinate as the first BVP.
 6. The method of claim 1,wherein, when a constrained reference area of the current block isinaccessible from the memory, the determining whether the first RB isaccessible from the memory further comprises: determining whether thefirst RB is inside the constrained reference area of the current block;and when the first RB is determined to be inside the constrainedreference area of the current block, indicating that the first RB isinaccessible from the memory.
 7. The method of claim 6, wherein thedetermining the second RB further comprises: when the first RB isdetermined to be inside the constrained reference area of the currentblock, determining the second RB to be outside and close to theconstrained reference area according to a first size of the constrainedreference area and a second size of the current block.
 8. The method ofclaim 7, wherein the first size of the constrained reference area is amultiple of a third size of CTU.
 9. An apparatus, comprising aprocessing circuitry configured to: decode prediction information for acurrent block in a current coded picture that is a part of a coded videosequence, the prediction information indicating an intra block copy(IBC) mode and a first block vector predictor (BVP) used for the currentblock, the first BVP pointing to a first reference block (RB); determinewhether the first RB is accessible from a memory; when the first RB isdetermined to be inaccessible from the memory, determine a second RBthat is accessible from the memory according to at least a median pixelvalue of a bit depth of the coded video sequence; include a second BVPin a BVP candidate list, the second BVP pointing to the second RB; andreconstruct the current block based on the BVP candidate list.
 10. Theapparatus of claim 9, wherein the processing circuitry is furtherconfigured to: when an allowed reference area of the current block isaccessible from the memory, determine whether the first RB is outsidethe allowed reference area of the current block; and when the first RBis determined to be outside the allowed reference area of the currentblock, indicate that the first RB is inaccessible from the memory. 11.The apparatus of claim 10, wherein the allowed reference area of thecurrent block includes one of: a reconstructed area in a current codingtree unit (CTU); and the reconstructed area in the current CTU and apart of an adjacent CTU that is on a left side of the current CTU. 12.The apparatus of claim 10, wherein the processing circuitry is furtherconfigured to: determine whether at least one available RB is found inthe allowed reference area of the current block; when the at least oneavailable RB is found in the allowed reference area of the currentblock, determine the second RB according to one of the at least oneavailable RB that is nearest to the first RB; and when the at least oneavailable RB is not found in the allowed reference area of the currentblock, determine the second RB according to a default RB or thepredefined pixel value.
 13. The apparatus of claim 12, wherein, when acoordinate of the first BVP is (x, y), a top-left coordinate of thedefault RB is one of an origin point (0, 0) of the current codedpicture, a boundary point (x, 0) of the current coded picture that hasthe same horizontal coordinate as the first BVP, and a boundary point(0, y) of the current coded picture that has the same verticalcoordinate as the first BVP.
 14. The apparatus of claim 9, wherein theprocessing circuitry is further configured to: when a constrainedreference area of the current block is inaccessible from the memory,determine whether the first RB is inside the constrained reference areaof the current block; and when the first RB is determined to be insidethe constrained reference area of the current block, indicate that thefirst RB is inaccessible from the memory.
 15. The apparatus of claim 14,wherein the processing circuitry is further configured to: when thefirst RB is determined to be inside the constrained reference area ofthe current block, determine the second RB to be outside and close tothe constrained reference area according to a first size of theconstrained reference area and a second size of the current block. 16.The apparatus of claim 15, wherein the first size of the constrainedreference area is a multiple of a third size of CTU.
 17. Anon-transitory computer-readable storage medium storing a program, whenexecuted by at least one processor, causes the at least one processor toperform a method comprising: decoding prediction information for acurrent block in a current coded picture that is a part of a coded videosequence, the prediction information indicating an intra block copy(IBC) mode and a first block vector predictor (BVP) used for the currentblock, the first BVP pointing to a first reference block (RB);determining whether the first RB is accessible from a memory; when thefirst RB is determined to be inaccessible from the memory, determining asecond RB that is accessible from the memory according to at least amedian pixel value of a bit depth of the coded video sequence; includinga second BVP in a BVP candidate list, the second BVP pointing to thesecond RB; and reconstructing the current block based on the BVPcandidate list.
 18. The non-transitory computer-readable storage mediumof claim 17, wherein the stored program further causes the at least oneprocessor to perform the method comprising: when an allowed referencearea of the current block is accessible from the memory, determiningwhether the first RB is outside the allowed reference area of thecurrent block; when the first RB is determined to be outside the allowedreference area of the current block, indicating that the first RB isinaccessible from the memory; determining whether at least one availableRB is found in the allowed reference area of the current block; when theat least one available RB is found in the allowed reference area of thecurrent block, determining the second RB according to one of the atleast one available RB that is nearest to the first RB; and when the atleast one available RB is not found in the allowed reference area of thecurrent block, determining the second RB according to a default RB orthe predefined pixel value.
 19. The non-transitory computer-readablestorage medium of claim 18, wherein the allowed reference area of thecurrent block includes one of: a reconstructed area in a current codingtree unit (CTU); and the reconstructed area in the current CTU and apart of an adjacent CTU that is on a left side of the current CTU. 20.The non-transitory computer-readable storage medium of claim 17, whereinthe stored program further causes the at least one processor to performthe method comprising: when a constrained reference area of the currentblock is inaccessible from the memory, determining whether the first RBis inside the constrained reference area of the current block; when thefirst RB is determined to be inside the constrained reference area ofthe current block, indicating that the first RB is inaccessible from thememory; and determining the second RB to be outside and close to theconstrained reference area according to a first size of the constrainedreference area and a second size of the current block.